Cellular Auxin Homeostasis: Gatekeeping Is Housekeeping Michel Ruiz Rosquete, Elke Barbez, Jürgen Kleine-Vehn  Molecular Plant  Volume 5, Issue 4, Pages 772-786 (July 2012) DOI: 10.1093/mp/ssr109 Copyright © 2012 The Authors. All rights reserved. Terms and Conditions

Figure 1 Auxin Biosynthesis. Tryptophan (TRP)-dependent and -independent auxin biosynthesis pathways are indicated. The TRP-derived pathways are highlighted in yellow (IAOx), blue (IAM), green (TAM), and red (IPA). The functional interdependence or redundancy of the proposed auxin biosynthesis routes is still a matter of debate. The pathways proposed here are largely based on the updated depiction in Mashiguchi et al. (2011). Auxin biosynthetic intermediates are shown in black, auxin biosynthesis enzymes in red, and internal and external triggers that regulate enzyme expression in gray. The identity of the enzymes catalyzing some of the suggested reactions remains elusive. AAO, ACETALDEHYDE OXIDASE; AMI1, AMIDASE 1; CYP79B2/3, CYTOCHROME P450, FAMILY 79, SUBFAMILY B, POLYPEPTIDE 2/3; IAA, Indole-3-acetic acid; IAAld, Indole-3-acetaldehyde; IAM, Indole-3-acetamide; IAN, Indole-3-acetonitrile; IAOx, Indole-3-acetaldoxime; IPA, Indole-3-ylpyruvic acid; NIT, NITRILASE; TAA1, TRYPTOPHAN AMINOTRANSFERASEOFARABIDOPSIS; TAM, Tryptamine; TDC, TRYPTOPHAN DECARBOXYLASE; TRP, Tryptophan. Molecular Plant 2012 5, 772-786DOI: (10.1093/mp/ssr109) Copyright © 2012 The Authors. All rights reserved. Terms and Conditions

Figure 2 Conversion and Inactivation of Auxin. Active free IAA levels can be modulated via conversion into IBA or IAA conjugation to amides or esters. Both conjugation and conversion to IBA might provide a mechanism for the temporary storage of reversibly inactivated auxins. Free IAA and amide conjugates (irreversible), such as IAA-Asp and IAA-Glu, eventually undergo oxidation-based degradation. Besides catabolism and storage of auxin, conversion into IBA and IAA conjugation could also be implicated and important for auxin signaling and transport. Asp, Aspartic acid; Glu, Glutamic acid; GH3, GRETCHEN HAGEN 3; IAA, Indole-3-acetic acid; ILL, IAA-LEUCINE RESISTANT (ILR)-LIKE; IBA, Indole-3-butyric acid; IBR, INDOLE-3-BUTYRIC ACID RESPONSE; UGT, UDP-GLUCOSYL TRANSFERASE. Molecular Plant 2012 5, 772-786DOI: (10.1093/mp/ssr109) Copyright © 2012 The Authors. All rights reserved. Terms and Conditions

Figure 3 Carrier-Driven Cellular Auxin Homeostasis. Due to the cytoplasmic pH, IAA is mainly deprotonated and hence negatively charged, thus requiring an auxin carrier to exit the cells. PIN1-type proteins (red) determine the directional auxin export from cells. ABCB transporters (blue) mainly facilitate non-directional auxin efflux, whereas AUX1/LAX (green) catalyzes its influx. Under certain conditions, ABCB4 and NRT1.1 (gray) also import auxin into the cell. The atypical PIN5 (inset) auxin carriers at the ER regulate auxin compartmentalization into the ER lumen and therefore affect cytoplasmic availability of auxin and its metabolism. Gray arrows indicate the respective auxin transport direction of the depicted auxin carriers. IAA, Indole-3-acetic acid; AUX1, AUXIN RESISTANT 1; ER, Endoplasmic Reticulum; LAX, LIKE AUX1; ABCB, ATP-BINDING CASSETTE; NRT, HIGH-AFFINITY NITRATE TRANSPORTER; PIN, PIN-FORMED. Molecular Plant 2012 5, 772-786DOI: (10.1093/mp/ssr109) Copyright © 2012 The Authors. All rights reserved. Terms and Conditions

Figure 4 Distinct Polarization of PIN Proteins in Arabidopsis Roots. PIN proteins in the stele, such as PIN1, show preferential basal (root tip-oriented) polarization. In contrast, PIN2 in epidermal cells display an apical (shootwards) localization. PIN3 and PIN7 are largely non-polarly localized in gravity-sensing columella cells, but undergo polarization in response to gravity (see root illustration on the left). Polar PIN distributions correlate and determine the auxin flow direction. The presumptive spatial accumulation of auxin in the very root tip is depicted (see root illustration on the right). PIN, PIN-FORMED. Molecular Plant 2012 5, 772-786DOI: (10.1093/mp/ssr109) Copyright © 2012 The Authors. All rights reserved. Terms and Conditions

Figure 5 Cellular Levels of Active Auxin Are Spatio-Temporally Determined by a Complex Regulatory Network. Biosynthesis, conjugation, oxidation, and transport of auxin (black boxes) jointly define the level of active/free auxin in the cells. Cellular auxin levels have an immediate output via auxin signaling components (yellow boxes). The phytohormonal crosstalk (outer circle) modulates and eventually integrates auxin metabolism, transport, and signaling. Molecular Plant 2012 5, 772-786DOI: (10.1093/mp/ssr109) Copyright © 2012 The Authors. All rights reserved. Terms and Conditions